Adhesion can be defined as the state in which two surfaces are held together by interfacial forces which may consist of valence forces, molecular bonding, or mechanical, interlocking action. Failure of the adhesive can result from the bond between the adhesive and one or both substrates being insufficient to withstand force applied to the adhered substrates so that the adhesive strips away from one or both substrates. Cohesive failure of the adhesive can occur when the internal strength of the adhesive is not as great as the force applied to it. In such failure, the adhesive may remain bonded to both substrates. As an adhesive is subject to increasing force, its behavior depends on the elasticity of the adhesive.
Gluing materials together with conventional adhesives has traditionally involved either relatively stiff adhesives such as epoxies, or elastic adhesives such as silicon adhesives. As illustrated in the force vs. extension curves of FIG. 1, when pulling two surfaces glued together with a short molecule, the pulling force increases rapidly with only a little extension of the molecule. A perfect stiff adhesive would be a short molecule bound to each surface by strong (that is, covalent or ionic) bonds and the molecules of the adhesive itself would be held together with strong bonds. Thus the break strength of each adhesive molecule would be the force required to break a strong bond: of the order of one nanoNewton, which is estimated by dividing one electron volt by an extension of one angstrom. For a material with many strongly bound molecules in parallel, the microscopic tensile strength is expected to be of the order of several gigapascals. This is the order of magnitude for the breaking force of strong polymers such as Kevlar [AI-Hassani, S. T. et al, Strain rate effects on GRP, KRP and CFRP composite laminates. Key Eng. Mater. 141, 427-452 (1998); Greenwood, J. H. et al, Compressive behavior of Kevlar 49 fibres and composites. J. Mater. Sci. 9, 1809-1814 (1974)]. The fracture toughness of such materials is rather small, however, even though the forces are large. This can be understood by considering the area under the force vs. extension curve shown in FIG. 1, which is the energy required to break the material. Because those stiff materials have a small elastic strain, the extension over which the force must be exerted until it breaks is small. Therefore, the area under the curve, or the energy required to break the material, is small.
In contrast to this behavior, the idealized curve for an elastic fiber made of long molecules shows that the force increases slowly as the elastic material is stretched to the point at which the elastic limit is reached, as also illustrated in FIG. 1. [Lu, H. et al, Unfolding of titin immunoglobulin domains by steered molecular dynamics simulation. Biophysics, J. 75, 662-671 (1988); Slater, G. W. et al, Construction of approximate entropic forces for finitely extensible nonlinear elastic (FENE) polymers. Macromol. Theory Simulat. 3, 695-704 (1994).] Then the force increases rapidly for further extension until it breaks. As with stiff adhesives, this break will also occur at a force of the order of one nanoNewton per molecular chain, assuming each chain is bound to each surface with a strong bond and is itself held together by strong bonds. Contrary to the case of short, inelastic molecules, the pulling force must be applied over much larger extensions. Therefore, the area under the force vs. extension curve would be larger, as shown in FIG. 1, and thus more energy would be needed to break the material. Unfortunately, the technology does not at present exist for making such an idealized elastic material. Real elastic materials such as rubbers have tensile strengths that correspond to breaking forces per molecule of the order of 0.1% of the theoretical maximum.
Natural materials are renowned for strength and toughness not possessed by man made materials. [Qin, X., Coyne et al, A novel natural copolymer: A collagenous molecule from mussel byssus contains silk fibroin-like domains. Am. Zool 37, 125A (1997)] Spider dragline silk has a breakage energy per unit weight two orders of magnitude greater than high tensile steel. [Hinman, M. et a, in Biomedical Materials (eds Viney, C. et al 25-34 (Materials Research Soc., Pittsburgh, 1993); Heslot, H. Artificial fibrous proteins: A review. Biochimie (Paris) 80, 19-31) (1998)] and is representative of many other strong natural fibers. [Waite,J. H. et a, The peculiar collagens of mussel byssus. Matrix Biol. 17, 93-106 (19998); Vollrath, F. et al, Modulation of the mechanical properties of spider silk by coating with water. Nature 340, 305-307 (1989); Qin, X. X. et al, Tough tendons, Mussel byssus has collagen with silk-like domains. J. Biol. Chem. 272, 32623-32627 (1997)]
Titin, the giant sarcomeric protein of striated muscle, is capable of massive length gains, allowing the muscle to be overstretched without irreversible damage to the sarcomere. Rief et al used a single-molecule atomic force microscope to repeatedly stretch individual titin molecules to elongate them. [Rief, M. et al, Reversible unfolding of individual titin immuno-globulin domains by AFM. Science, 276, 1109-1112 (1997)] Rief et at found that for larger extensions, the force vs. extension curves typically exhibited a sawtooth-like discontinuity which they hypothesized might reflect the successive unraveling of individual domains of a single titin molecule, unfolding one domain at a time.
The abalone shell, a composite of calcium carbonate plates sandwiched between organic material, is 3000 times more fracture resistant than a single crystal of the pure mineral. [Currey, J. D. Mechanical properties of mother of pearl in tension. Proc. R. Soc. Lond. B 196, 443-463 (1997); Jackson, A. P. et al, The mechanical design of nacre. Proc. R. Soc. Lond. B 415-440 (1988)] The organic component, comprising just a few percent of the composite by weight [Watable, N. et al (eds.) The Mechanisms of Bimineralization in Invertebrates and Plants (Univ. South Carolina Press, Columbia, SC., 1976)], is thought to hold the key to nacre""s fracture toughness. [Weiner, S. Organization of extracelluarly mineralized tissues: a comparative study of biological crystal growth. CRC Crit. Rev. Biochem. 20, 365-408 (1986); Jackson, A. P. et al, A physical model of nacre. Composites Sci. Technol. 36, 255-266 (1989)]
Nacre is the scientific name given the xe2x80x9cmother of pearlxe2x80x9d on the inside of the abalone shell. A matrix protein, named Lustrin A, from the nacreous layer of the shell and pearl produced by the abalone, Haliotis rufescens, was cloned and its cDNA coding characterized. [Shen, X. et al, Molecular cloning and characterization of Lustrin A, a matrix protein from shell and pearl nacre of Haliotis rufescens. J. BioL Chem. 272, 32472-32481 (1997)] This protein is found between the mineral plates in the abalone shell and participates in holding these plates together. The complete amino acid sequence of Lustrin A was reported by Shen et al, revealing a shell matrix protein with a repeating modular structure. A schematic representation of the modular structure as elucidated by Shen et al is shown in FIG. 2 where cysteine-rich modules (C1-C9) and proline-rich modules (P1-P8) are arranged in tandem and repeated nine and eight times, respectively, in the N-terminal two-thirds of Lustrin A.
The reason for nacre""s fracture resistance does not appear to be simply its lamination with an organic material, but the modular nature of the organic material. Ceramics laminated with organic material are more fracture resistant than non-laminated ceramics [Jackson, A. P. et al, supra; Clegg, W. J. et al, A simple way to make tough ceramics. Nature 347, 455-457 (1990)], but synthetic materials made of interlocking ceramic tablets bound by a few weight per cent of ordinary adhesives do not have a toughness comparable to nacre. [Almqvist, N. et al, Methods for fabricating and characterizing a new generation of biomimetric materials. Mater. Sci. Eng. C 7(1), 37-43 (1999)]
The present invention provides a material having the advantages of both short molecule fibers or adhesives and long molecule fibers or adhesives, without their individual drawbacks. The material displays a force vs. extension curve that rises to a large force quickly, but then dissipates energy to maintain that force over large extensions. The material is a modular, energy-dissipating material, the modules serving a protective role to prevent catastrophic failure of the material. The modules comprise folded subunits or domains, each domain intraconnected by sacrificial bonds that unfold to dissipate energy in a stepped fashion at forces below that necessary to break the backbone of the material and, with adhesive material, below that necessary to break the bonds that fasten the material to surfaces being glued. Modular adhesives of this invention also serve to give more rigidity than conventional elastic adhesives because for the same length backbone, the modular adhesive molecule is effectively much shorter because most of its length is folded into compact modules.
The invention encompasses the use of modular energy-dissipating material, including that recovered from natural sources, as adhesives materials. It can be used to bond opposing surfaces together, by applying it to the opposing surfaces while the domains are unfolded, and then causing the domains to fold to pull the surfaces together. In particular embodiments, the opposing surfaces can have different bonding characteristics, the polymer structure of said modular material being linear and wherein its opposite ends selectively bond to the different surfaces.
The invention also encompasses the use of modular energy-dissipating material, including that recovered from natural sources, as fibers and in composites, and in a wide variety of other non-adhesive uses, for example as reinforcements, such as for tires and sports equipment, as armor and as elastomeric sealants for repairs.